This invention relates to semiconductor devices and more specifically relates to a novel diode structure for application to automotive structures.
Semiconductor devices are commonly used in automotive applications such as in three-phase bridge designs for automotive alternators. These are generally mounted in the hostile environment of xe2x80x9cunder-hoodxe2x80x9d locations. Despite this hostile environment (heat, vibration, shock forces, etc.), high reliability and thermal efficiency are key requirements.
Button diodes of a well-known type are commonly used for automotive alternators. Such button diodes have a cylindrical shape with a cylindrical conductive outer rim and flat top and bottom electrodes which are insulated from the rims, and define the cathode and anode electrodes of the device. These button diodes are commonly mounted on two separate heat sink sections which form the positive and negative d-c rails for the output of the three-phase bridge circuit. The cathode electrodes of three of the diodes are mounted on the negative bus and the three other button diodes are flipped over with their anodes mounted on the positive bus.
Conventional button diodes have been found to be unreliable for alternator outputs above about 2 kW, which are needed for many modern automotive alternator applications. Further, the upward-facing or free electrode (anode or cathode) of the button diode requires a separate clip connector for connecting one of the circuit a-c output leads to said free electrode of the button diode.
It would be desirable to provide a diode structure which is reliable for operation in an alternator application at an output power in excess of 2 kW, and which can be applied to existing alternator structures and heat sinks.
In accordance with the invention, a novel alternator diode is provided which can directly replace a button diode in an existing alternator structure, but can be reliably used for higher output power.
The novel diode of the invention consists of a diode package of a TO 220 type outline and which has an internal diode die having one electrode mounted to a first lead frame section and its other electrode wire bonded to an extending second lead frame tab which is insulated from the first section. The die and lead frame sections are over-molded with a conventional plastic housing, with the bottom of the first lead frame section exposed for surface mounting and with the second and tab section of the lead frame extending through the side wall of the housing. The end of the second section is preferably forked to define an easy screw or bolt connector connection to the common a-c connection. The exposed bottom surface of the first section and the extending tab are preferably metallized with a solderable finish. The die within the package can be mounted with the anode side up or down to define the diode to be connected to the positive or negative d-c bus respectively.
In one embodiment of the invention, the diode may be a Zener diode which, at 25xc2x0 C. has a Zener voltage of 28/33 volts; a forward voltage drop of 1 volt at 100 amperes; an R junction-case of 0.6xc2x0 C./W; a lead current rating greater than 75 amperes and an IAV (180xc2x0 Rect.) of 80 amperes at a case temperature of 125xc2x0 C. This rating is suitable for many alternator designs with outputs available above the 3 kW level. Further, the novel structure provides lower assembly costs and, critically, more reliable operation at higher power in the hostile xe2x80x9cunder-hoodxe2x80x9d environment.